1. Field of the Invention
The present invention relates to a method and an apparatus for measuring an optical property of a fluorescent sample.
2. Description of the Related Art
Today, paper and fabrics are often treated by FWA (Fluorescent Whitening Additives) and the effect of fluorescence to the observed whiteness or color of such products (referred to as “fluorescent sample” hereinafter) is not negligible. So that, method and an apparatus for measuring the optical property of those taking the effect of fluorescence into account is required.
Generally a visual property of a reflecting sample is expressed relatively to the perfect white and based on the total spectral radiance factor B(λ) which is the ratio of light emitted from the sample to that from the perfect reflecting diffuser in the identical illuminating and receiving condition.
A color of fluoresced light is observed as a light source color alone, in case of a fluorescent sample, however, the fluoresced light is added to the reflected light and the color is observed as objective color. That is, the light emitted from the fluorescent sample is the sum of the reflected light and the fluoresced light, and accordingly the total spectral radiance factor B(λ) of the fluorescent sample is also given as the sum of reflection spectral radiance factor R(λ) and the fluorescent spectral radiance factor F(λ) which are the ratios of light reflected and fluoresced from the sample respectively to the light reflected from the perfect reflecting diffuser in the identical illuminating and receiving condition as expressed by Equation 1.B(λ)=R(λ)+F(λ)  (1)
Since the above mentioned perfect reflecting diffuser has no fluorescence and the reflectivity of which has no dependence on the wavelength, abovementioned total spectral radiance factor B(λ), reflection spectral radiance factor R(λ) and fluorescent spectral radiance factor F(λ) are equivalent to the ratios of the light emitted, reflected and fluoresced from the sample respectively to the illumination light with a suitable proportional coefficient. The color of a fluorescent sample is observed as an objective color, and accordingly is related to the total spectral radiance factor B(λ), from which the calorimetric values are derived.
CIE (International Committee of Illumination) defines spectral intensity distributions of several standard illuminations for colorimetry such as Illuminant D65, D50, D75 (daylight), Illuminant A (incandescent lamp), Illuminant F's, and Illuminant C. For the evaluation of fluorescent samples, Illuminant D65 or Illuminant C are generally used. The spectral excitation and fluorescence characteristics of fluorescent material is expressed by the Bi-spectral Luminescent Radiance Factor (referred to as “BLRF” hereinafter) F(μ,λ) which is the matrix data showing the intensity of the fluoresced light at wavelength λ excited by monochromatic light of a unit intensity at wavelength μ.
An example of abovementioned matrix data is shown in FIG. 8 where the cross-section along the fluorescence wavelength λ expresses the spectral excitation efficiency for fluorescing at wavelength λ while the cross-section along the excitation wavelength μ expresses the spectral intensity of fluoresced light excited at wavelength μ. Accordingly, a sample containing fluorescent substance of the bi-spectral luminescent radiance factor F(μ,λ) has the fluorescent spectral radiance factor F(λ) expressed by Equation (2), where the proportional coefficient is neglected, when illuminated by the light of the spectral intensity I(λ).F(λ)=F(μ,λ)·I(μ)dμ/I(λ)  (2)That is F(λ) is obtained as the ratio of convolution of the spectral intensity I(μ) of the illumination and the bi-spectral luminescent radiance factor F(μ,λ) to I(λ).
As indicated by Equation (2), the fluorescent spectral radiance factor F(λ) depends on the spectral intensity I(μ) of the illumination. Accordingly, the total spectral radiance factor B(λ) being the sum of the reflection spectral radiance factor R(λ) which itself doesn't depends on the spectral intensity I(μ) of the illumination and the fluorescent spectral radiance factor F(λ), and the calorimetric values derived therefrom also depend on I(μ).
As the result, the spectral intensity I(μ) of the illumination (referred to as “illumination for testing” hereinafter) need to be specified when evaluating the optical property of a fluorescent sample and for the accurate measurement, the spectral intensity I(μ) of the illumination of a measuring apparatus need to be same as that of the specified illumination for testing. However, it is difficult and expensive to realize such an illumination of the same spectral intensity as that of standard illuminant D65 or C generally used as the illumination for testing.
Alternatively, the total spectral radiance factor B(λ) or the fluorescent spectral radiance factor F(λ) can be calculated using Equation (2) with the measured bi-spectral luminescent radiance factor F(μ,λ) or bi-spectral radiance factor B(μ,λ) of the sample and the spectral intensity I(μ) of the illumination for testing given as numerical data. Here, similarly to the bi-spectral luminescent radiance factor F(μ,λ), the bi-spectral radiance factor B(μ,λ) is the-matrix data showing the intensity of the total emission which is the sum of the fluoresced light at wavelength λ excited by monochromatic light of a unit intensity at wavelength μ and the reflected light. The total spectral radiance factor B(λ) is obtained as the ratio of the convolution of the spectral intensity I(μ) of the illumination and the bi-spectral radiance factor B(μ,λ) to the I(λ).B(λ)=∫B(μ,λ)·I(μ)dμ/I(λ)  (2-1)
However, since the measurement of the bi-spectral luminescent radiance factor F(μ,λ) or the bi-spectral radiance factor B(μ,λ) requires a complicated and expensive bi-spectro-fluorimeter comprising two spectral units, one for illumination and the other for receiving, and long time for measurement, this method is not practical. Quality controls of products treated by FWA such as paper are performed generally using either of two simplified methods mentioned below.
<Gaertner and Griesser's Method>
As shown in FIG. 10, fluorescent sample 601 is placed at sample aperture 603 of integrating sphere 602 of measuring apparatus 600 for measuring an optical property. Light source 604 such as Xe flash lamp contains sufficient UV component and the light flux 605 from it passes through the aperture and enters integrating sphere 602. A UV cut filter 606 is inserted so as to partially block the optical path of flux, and the flux which passes through the UV cut filter has the UV component eliminated. The degree of insertion of UV cur filter 606 is adjustable so as to allow adjustment of the UV intensity in the illumination light. Flux 605 partly passing through UV cut filter 606 and entering integrating sphere 602 undergoes diffuse reflection within the sphere and forms diffuse light which illuminates the fluorescent sample 601, and the radiant light 607 emitted in a predetermined direction from the illuminated surface passes through the observation aperture and enters sample spectral unit 608 which detects the spectral intensity Sx(λ). Similarly, light flux 609 having the same intensity as the illumination light of fluorescent sample 601 enters monitoring optical fiber 610 so as to be directed to monitoring spectral unit 611 which detects the spectral intensity Mx(λ). Controller 612 calculates the total spectral radiance factor Bx(λ) from the spectral intensities Sx(λ) and Mx(λ) detected by spectral units 608 and 611.
A fluorescence standard containing fluorescent material having the excitation and fluorescence characteristics namely the bi-spectral luminescent radiance factor F(μ,λ) identical or similar to that of the sample to be measured and given a colorimetric value such as CIE whiteness under the specified illumination for testing is used to determine the degree of insertion of UV cut filter 606. The fluorescence standard is measured by measuring apparatus 600, and the UV intensity is corrected by adjusting the degree of insertion of UV cut filter 103 so as to match the value of CIE whiteness calculated from the obtained total spectral radiance factor Bx(λ) to the CIE whiteness given to the fluorescence standard.
Gaertner and Griesser's method is mechanically complicated and unreliable, and also requires complicated and time-consuming operation, that is, measurements and movements of UV cut filter need to be repeated until the measured colorimetric value, CIE whiteness for example, agrees the given value. This method results the single specific colorimetric value, CIE whiteness in this case, compatible to that under specified illumination for testing, however from the principle, the multiple colorimetric values, the CIE whiteness and Tint value for example, or the total spectral radiance factor Bx(λ) are not compatible simultaneously.
<Method of U.S. Pat. No. 5,636,015>
While Gaertner and Griesser's method modifies the UV content in the illumination first and modifies the total spectral radiance factor Bx(λ) as the result, this method numerically synthesizes the total spectral radiance factor Bx(λ) first and synthesizes the illumination of the spectral intensity necessary for the Bx(λ) as the result. As shown in FIG. 11, integrating sphere 702 of measuring apparatus 700 is provided with a first illuminator 704 emitting light flux containing a UV component and a second illuminator 705 emitting light flux containing no UV component. Measuring apparatus 700 is further provided with a first spectral unit 709 detecting the spectral intensity of emitted light 708 from the fluorescent sample 701 placed at sample aperture 707 and a second spectral unit 712 detecting the spectral intensity of light 710 of the illumination conducted through optical fiber 711, and control unit 713. The fluorescent sample 701 is illuminated by first and second illuminators consecutively and the spectral intensities Sx1(λ) and Sx2(λ) of emitted light from said sample and the spectral intensities Mx1(λ) and Mx2(λ) of the illumination light are respectively detected. The total spectral radiance factors Bx1(λ) and Bx1(λ) corresponding to the illuminations by first and second illuminators are obtained from Sx1(λ), Sx2(λ), Mx1(λ), and Mx2(λ) and thus, the total spectral radiance factor Bxc(λ) is synthesized by linearly combining Bx1(λ) and Bx2(λ) with the weight W(λ) as shown in Equation (3)Bxc(λ)=W(λ)·Bx1(λ)+(1−W(λ))·Bx2(λ)  (3)
Similar to Gaertner and Griesser's method, abovementioned weight W(λ) for each wavelength λ is determined using a fluorescence standard containing fluorescent material having the excitation and fluorescence characteristics namely the bi-spectral luminescent radiance factor F(μ,λ) identical or similar to that of the sample to be measured and given a total spectral radiance factors Bs(λ) under the specified illumination for testing. That is, weight W(λ) is so determined for each wavelength λ numerically that the synthesized total spectral radiance factor Bxc(λ) by Equation (3) matches the given total spectral radiance factors Bs(λ) under the specified illumination for testing.
This method is equivalent to respectively performing the correction of the UV content in the illumination by Gaertner and Griesser's method for the total spectral radiance factor Bx(λ) at each wavelength as the target instead of the single colorimetric value. Since this method gives the total spectral radiance factors Bxc(λ) of the sample comparable to Bs(λ) under the specified illumination for testing, it has an advantage that all colorimetric values derived therefrom are also comparable to those under the specified illumination. Although this method eliminates many shortcomings of Gaertner and Griesser's method such as the mechanical complicacy, lack of reliability, and complicated and time-consuming operation, it still requires a fluorescence standard and errors due to the difference between the spectral intensity of the illumination at the time of UV correction and that at the time of sample measurement thereafter still remains.
If paper is treated by FWA, colors printed thereon are affected by fluorescence of the paper. Since the amount of excitation light reaching the paper substrate depends on the spectral transmittance of ink-covering the paper substrate, the spectral excitation-fluorescence characteristics (spectral excitation efficiency and spectral fluorescence intensity) of the printed paper depend on not only the spectral excitation-fluorescence characteristics of the paper but also on the spectral transmittance and the dot area (relative area covered by ink) of the ink on the area of paper. If paper is printed with two or more different inks, the paper is covered with those inks and the superposition of those, and accordingly, the spectral excitation-fluorescence characteristics of the measuring area depends on the spectral transmittance and the dot area of each of inks and the superposition of those.
Ink with the transmittance independent on wavelength doesn't change the relative spectral intensity of the illumination light reaching the paper substrate and equally influences to the spectral integrated excitation efficiency of the illumination synthesized by method of U.S. Pat. No. 5,636,015 and to that of the illumination for testing. Accordingly, the synthesized total, spectral radiance factor Bxc(λ) of the printed paper is comparable to that to be obtained under the specified illumination for testing although they are different from those of unprinted paper. Here, the spectral integrated excitation efficiency E(λ) expressed by Equation (4) is the excitation efficiency for fluorescence at wavelength excited by the whole illumination.E(λ)=∫Q(μ,λ)·I(μ)dμ  (4)where Q(μ,λ) is the bi-spectral excitation efficiency, that is the excitation efficiency for fluorescence at wavelength λ excited by light of a unit intensity and of bandwidth dμ at wavelength μ.
As described above, both simplified methods (Gaertner and Griesser's method and method of U.S. Pat. No. 5,636,015) need the fluorescence standard. Since the fluorescence standard made of the same material as sample to be measured such as paper or fabric and containing the same fluorescent material as that contained in the sample is unstable and requires considerable cares for controlling the change due to the aging and for the renewal. Further, errors due to the change of the spectral intensity of the illumination after the UV correction is inevitable, and as the result, frequent UV readjustments are required for avoiding these errors. From these, a method and an apparatus for measuring fluorescent sample free from a fluorescence standard and a UV correction using the fluorescence standard are required.
Printing generally applies four primary inks (YMCK) on fluorescent paper and all inks except K (black) ink have the wavelength-dependent transmittances. That is, for most of printing material, abovementioned simplified methods do not provide an accurate colorimetric measurement taking the effect of fluorescence in account. Thus, a method and an apparatus for measuring printed colors on fluorescent paper taking not only the fluorescence of paper but also the spectral transmittance and the dot area of each of inks and the superposition of those in account are required. Said method and apparatus are further required to be free from a fluorescence standard and a UV correction.
The objective of the present invention is to provide a method and an apparatus for measuring the optical property of a fluorescent sample comparable to that under the specified illumination for testing without a fluorescence standard and a bothersome UV correction using the fluorescence standard.